## Sunday, September 3, 2023

### A Yoneda week

The 35th International Olympiad in Informatics in Szeged, Hungary was the main event of this week (problems, results, top 5 on the left, day 1 broadcast, day 2 broadcast). Tingqiang Xu and Siyuan Cheng were miles ahead of everybody else (who still did great :)), and only 1 point ended up deciding the overall winner. Congratulations to them but also to all medalists! My inevitable slide in the Hall of Fame continued, seemingly by just 1 place though.

According to the problem page, three out of six problems were created by the amazing team of square1001 and E869120. Creating just one IOI-quality problem is an amazing achievement requiring great ideas and weeks (if not months) of work, so it is hard for me to even imagine creating three in the same year. Well done!

Codeforces ran Pinely Round 2 just a few hours after the first IOI day was over (problems, results, top 5 on the left, my screencast, discussion, analysis). I wonder who is the highest-placed participant who took part in both :) Among the participants well past their IOI days, tourist got his first place using just under an hour out of the three hours available. It could all change at the end of the round, as both last problems were not completely unsolvable and saw a lot of last-minute submissions, but nobody was able to get all details exactly right. Congratulations to tourist!

Problem F did not require one to come up with complicated ideas, but instead required some accuracy "unwrapping the present" to arrive at the solution, and it was nice that the solution turned out quite easy to implement: you are given an array of at most 10000 integers, each between 0 and 260. In one operation, you split the array in the middle into two parts, compute the bitwise xor of each part, and discard the part where the bitwise xor is smaller. In case they are equal, you may discard either part. After doing this operation several times, you have just one number remaining. Which positions in the initial array could this number correspond to?

You might have noticed that the address of this blog has changed to https://blog.mitrichev.ch/. I am going to use this address going forward, even though the old address will keep redirecting to the new one for the foreseeable future. So, welcome to the new home!

It is also a good opportunity to write about my online presence in general. I do not have accounts in the common social networks, such as Facebook, Twitter/X, LinkedIn, TikTok, Instagram, etc. Of the social network-like things I mainly have this blog, my Youtube channel, and my Codeforces account.

Thanks for reading, and check back soon! I will try to post updates about the AWTF during the week.